Carbon dioxide has a boiling point of −78 degrees Celsius, with the result that carbon dioxide is typically present on earth as a gas. The carbon dioxide molecule includes two oxygen atoms which are attached by double bonds to a single carbon atom. Carbon dioxide is relatively non-reactive and is not flammable.
Carbon dioxide is derived from natural sources such as the oxidation of organic material, fermentation processes and as a product of respiration by aerobic organisms. Carbon dioxide is also derived from man-made sources such as the burning of fossil fuels (i.e., hydrocarbons) and as a by-product of industrial processes. Carbon dioxide is consumed by plants during photosynthesis.
Carbon dioxide is present in the Earth's atmosphere at a low concentration. However, carbon dioxide is the most abundant “greenhouse gas”, a group of atmospheric gases which includes carbon dioxide, methane, nitrous oxide and fluorocarbons. It is generally believed that greenhouse gases may contribute to climate change and global warming.
The Kyoto Protocol is an agreement made under the United Nations Framework Convention on Climate Change (UNFCCC), which agreement entered into force on Feb. 16, 2005. Countries which have ratified the Kyoto Protocol have agreed to control and/or reduce emissions into the atmosphere of carbon dioxide and other greenhouse gases.
Many countries have ratified the Kyoto Protocol, with notable exceptions being the United States and Australia. In any event, there appears to be a general consensus throughout the world that controlling and/or reducing emissions of carbon dioxide and other greenhouse gases is generally desirable.
Carbon dioxide exhibits a relatively high solubility in water. The solubility of carbon dioxide in water does, however, decrease with increasing temperature.
In an aqueous environment carbon dioxide is typically present as dissolved carbon dioxide, as carbonic acid, as bicarbonate ions and as carbonate ions in varying amounts to establish equilibrium as a “carbonate system” or “carbonic acid system”, depending upon the concentrations of the various carbon dioxide species and the pH of the aqueous environment. Carbon dioxide typically dissolves and dissociates in water in several steps according to the following formulae:CO2(gas)⇄CO2(aqueous)  (1)CO2(aqueous)+H2O⇄H2CO3(aqueous)   (2)H2CO3⇄H++HCO3−  (3)HCO3−⇄H++CO32−  (4)
As the aqueous environment becomes less acidic and more basic, increasing amounts of carbonic acid (H2CO3), bicarbonate ion (HCO3−) and carbonate ion (CO32−) are formed in a stepwise manner as carbon dioxide dissolves and dissociates in the aqueous environment.
The presence of carbonate ions in an aqueous environment will tend to cause the formation and precipitation of carbonate compounds as carbon dioxide species from cations such as calcium (Ca2+) and magnesium (Mg2+) which are present in the aqueous environment, according to the following exemplary formulae:CO32−+Ca2+→CaCO3  (5)CO32−+Mg2+→MgCO3  (6)
This precipitation of carbonate compounds is reversible to produce carbonate ions as the aqueous environment becomes increasingly acidic, according to the following general formulae:CaCO3+2H+→Ca2++CO32−  (7)MgCO3+2H+→Mg2++CO32−  (8)
Industrial plants and processes often generate significant amounts of carbon dioxide species, including carbon dioxide, carbonic acid, bicarbonate ions, carbonate ions and carbonate compounds, due to the combustion of fossil fuels and/or as a by-product of the processes being performed.
For example, power plants may generate carbon dioxide species as a result of the combustion of fossil fuels such as coal and oil. Other facilities may generate carbon dioxide species due to the combustion of fossil fuels and as a by-product of the various processes that may occur at such facilities.
Plants and facilities may also generate carbon dioxide species through the use of process water to perform processes, either due to dissolution of carbon dioxide in the process water or due to the presence of cations or carbonate compounds in the process water or in other materials which come into contact with the process water.
For example, the recovery of bitumen from a feed material such as oil sand involves a number of processes which may contribute carbon dioxide species to process water. First, the feed material may contain carbonate compounds which are transferred to the process water during processing of the feed material. Second, air which is used in separation vessels and/or flotation machines may contribute atmospheric carbon dioxide which dissolves in the process water. Third, air exposure at the surface of separation vessels such as thickeners etc. may contribute atmospheric carbon dioxide which dissolves in the process water. Fourth, if combustion gases are used to heat the process water, carbon dioxide contained in the combustion gases may dissolve in the process water.
The loading of carbon dioxide species in process water may ultimately result in undesirable carbon dioxide emissions into the atmosphere, and may also interfere with equipment and with processes that are performed using the process water. These potential problems are exacerbated if the process water is recycled and reused to perform such processes, in which case the amount of carbon dioxide species contained in the process water will typically increase each time the process water is used.
Although carbon dioxide emissions may be undesirable in the context of the general goal of limiting greenhouse gas emissions, carbon dioxide itself is useful for many purposes. Carbon dioxide in solid form (i.e., dry ice) is commonly used as a refrigerant and to carbonate soft drinks. Carbon dioxide is also useful as a non-flammable pressurized gas, as a fire extinguishing compound and as a solvent for organic compounds.
Carbon dioxide is also very useful in connection with hydrocarbon related processes. As one example, carbon dioxide is commonly injected under pressure into oil wells in connection with enhanced oil recovery (EOR) processes, wherein the carbon dioxide both serves to pressurize the producing formation and acts as a solvent for the oil to reduce its viscosity. As a second example, carbon dioxide is often used as a diluent to reduce the viscosity of bitumen to facilitate transportation of the bitumen through pipelines.
As a result, it would be desirable to remove carbon dioxide species from process water which has been used in an industrial process, thereby controlling carbon dioxide emissions which may result from the process water and reducing the loading of carbon dioxide species in the process water. It would also be desirable to collect carbon dioxide obtained from process water so that the carbon dioxide can be used for purposes for which the carbon dioxide may be suited.